Stresses and fracture probability in evacuated glazing

Evacuated glazing consists of two plane sheets of glass which are hermetically sealed around the edges and which contain a narrow evacuated space. An array of support pillars maintains the separation of the glass sheets under the influence of atmospheric pressure. The vacuum effectively eliminates gaseous heat transport through the structure, and radiative heat transport can be reduced to a low level by the incorporation of transparent, low emittance coatings on one or both of the inner glass surfaces. The application of evacuated glazing is in insulating windows. Evacuated glazing contains steady or long term stresses as a result of the combined effects of atmospheric pressure and temperature differentials. A method is developed for determining the magnitude of such stresses due to these separate influences. The stresses are calculated analytically, and by finite element modelling. The results of the calculations are validated by experimental measurements. These stresses are used with a model of delayed fracture in glass to estimate the probability of mechanical failure of evacuated glazing over long periods of time. It is shown that evacuated glazing which is well designed, manufactured and installed should be no more susceptible to mechanical failure than conventional double glazing, for similar operating conditions.